The chemical structure of polychlorinated biphenyls has been known for nearly 100 years. Commercial production was initiated in the United States in 1929 in response to the electrical industry's need for an improved dielectric insulating fluid for use in transformers and capacitors which would also provide increased fire resistant benefits.
The fire resistant nature of the polychlorinated biphenyls, combined with outstanding thermal stability made them excellent choices as hydraulic and heat transfer fluids alone or in formulations. They were also used to improve the waterproofing characteristics of surface coatings and offered many advantages to the manufacturer of carbonless copy paper, printing inks, plasticizers, special adhesives, lubricating additives, and vacuum pump fluids.
In the late 1960s, the first signs of polychlorinated biphenyls' potential environmental problems appeared. A Swedish biologist identified polychlorinated biphenyls as interference peaks in DDT determinations in the bodies of fish.
Since then, scientific investigations confirmed the presence of polychlorinated biphenyls in the environment in the United States. In the 1970s, Monsanto, as the sole U.S. manufacturer of polychlorinated biphenyls, voluntarily began a program to terminate sales of polychlorinated biphenyls for those applications that were likely to result in environmental contamination. By late 1976, Monsanto made arrangements to completely withdraw from the manufacture of polychlorinated biphenyls.
Because of their thermal and chemical stability and non-reactive nature, polychlorinated biphenyls tend to accumulate and persist in the environment. As the result of improved analytical techniques, polychlorinated biphenyls have been found in many places in the United States and have found their way into all levels of the food chain. Due to the known toxic effects of polychlorinated biphenyls, governmental actions have resulted in the control of the use, disposal, and production of polychlorinated biphenyls in nearly all world areas, including the United States. Thus, there is a need for an efficient and economic method for removing polychlorinated biphenyls from the environment.
It is known that polychlorinated biphenyls can be reduced to non-toxic biphenyls by catalytic dechlorination utilizing a hydrogenation catalyst and a hydrogen transfer donor. Typical hydrogen donors reported in the literature for this application are formic acid, hydrogen, ammonium formate, and sodium hypophosphite.
Potassium formate and sodium formate are the preferred hydrogen donors for the reduction of polychlorinated biphenyls. This is based on the fact that these salts are true hydrogen transfer agents, as no hydrogen gas is released in the course of the reactions. Hydrogen evolution is, in fact, the main limitation associated with the utilization of ammonium formate and other hydrogen donors, such as sodium hypophosphite.
Palladium supported on carbon is the most popular hydrogenation catalyst that is used for this application. There are studies using different supports for palladium, such as alumina, silica gel, calcium carbonate, and barium sulfate. There are also studies using homogeneous catalysts, such as palladium tetrakistriphenylphosphine.
It is also known that Raney nickel and platinum can be used as hydrogenation catalysts, either supported on other carriers or non-supported.
It has been demonstrated that the dechlorination of polychlorinated biphenyls, utilizing a hydrogenation catalyst and hydrogen transfer agent, is very sensitive to the concentration of water present in the system. The effect of water is attributed to the fact that water is an actual reactant in the hydrogen-transfer process and it has to compete with the other two substrates for an adsorption site on the catalyst surface. Further, it has been shown that the maximum rate is obtained when the water/formate molar ratio is about 3.
Catalytic dechlorination of pure samples of polychlorinated biphenyls, using palladium supported on carbon and potassium formate or sodium formate coupled with water as hydrogen transfer agents, proceeds readily in a variety of alcohols. However, the rate and extent of dechlorination diminishes when a polychlorinated biphenyl solution is taken from an actual environmental site.
Due to the presence of other contaminants in combination with polychlorinated biphenyls in the environment, the dechlorination reaction does not proceed to completion. It is known that one way to complete the dechlorination of an environmental sample is to add excess palladium supported on carbon. Thus, there is a need to develop an improved method of catalytic dechlorination of polychlorinated biphenyls that are generated from the environment. Such a method is needed to insure that the dechlorination of polychlorinated biphenyls proceeds readily to completion without the addition of excess palladium supported on carbon.